The effects of photosynthetically raised pH and light on some ciliated Protozoa in a eutrophic pond

Summary Some ciliated Protozoa (e.g. Loxodes magnus, L. striatus, Spirostomum teres, S. ambiguum and Frontonia leucas) are abundant during summer in the hypolimnion of a eutrophic pond in north‐west England but are absent from the epilimnion. The work described in this paper was begun with the aim of investigating the suggestion that high pH values caused by phytoplankton photosynthesis contributed to the exclusion of these ciliates from the epilimnion. In July 1973, phytoplankton photosynthesis and high pH were found only in the epilimnion, hence conditions were compatible with the above suggestion. Ciliates were, therefore, kept in the laboratory in hypolimnion water and were exposed to phytoplankton photosynthesis, both with and without pH increase. It was found that Loxodes died under both treatments hence there is no evidence that high pH is lethal to Loxodes. It seemed possible, therefore, that either light or toxins released by algae during photosynthesis are lethal to Loxodes. L. magnus was, therefore, exposed to light in the absence of phytoplankton (in filtered hypolimnion water) both in the laboratory and in the pond and it was found that light was lethal. High light intensities might, therefore, contribute to the exclusion of at least Loxodes species from the surface water of the pond, although other adverse factors are probably operative since Loxodes species do not migrate into the epilimnion at night.